Particularly in at least partly electrically operated vehicles, for example in electric vehicles and hybrid vehicles, provision is made of energy storage devices that store and provide the electrical energy required for driving operation.
The resistance of such an energy storage device constitutes an important characteristic value that serves in particular for determining a permissible power output. With increasing aging of the energy storage device, said resistance changes in this case, said resistance usually being determined on the test bed before the start-up of the energy storage device, that is to say at the so-called begin of life (BOL). After the initial start-up, the aging is crucially influenced by two criteria, namely the so-called cyclic aging and the so-called calendrical aging. However, the aging can be predicted only inaccurately on account of the different influencing factors. Moreover, nonuniform aging occurs at mutually different operating points.
In order to be able to determine e.g. the permissible power output accurately and reliably, it is desirable to ascertain the resistance as accurately as possible in each driving cycle of the vehicle and over the entire life cycle of the energy storage device.
In this case, it is desirable, in particular, to accurately determine the resistance already at the beginning of a driving cycle, in order to avoid an impermissible power release.
DE 102 57 588 B3 describes a method for predicting a voltage of a battery, in particular of a vehicle battery. The method described makes it possible to predict a voltage dip before it actually occurs on account of loading. For this purpose, inter alia, the dynamic internal resistance is used and firstly a filtered battery voltage and a filtered battery current are ascertained. Ohmic voltage drop across the dynamic internal resistance is ascertained from a difference current between the filtered battery current and a predefined load current. A predicted battery voltage is calculated from the filtered battery voltage, minus the ohmic voltage drop and the filtered polarization voltage. However, the document does not describe a specific way of determining the dynamic internal resistance.
WO 2006/057468 A1 discloses a method for estimating values describing present operating conditions of a battery. The method comprises estimating a state of charge in a battery, wherein the state of charge is encompassed by an internal state variable. Furthermore, the method comprises estimating a state of health in a battery, wherein the state of health is encompassed by an internal parameter. The document describes a Kalman filter, wherein the resistance of a battery cell is contained as a state variable or parameter in the Kalman filter.
One disadvantage of such a Kalman filter-based estimation is that inaccuracies resulting from the used model of an energy storage device can lead to an erroneous determination of the internal resistance of the energy storage device. By way of example, inaccuracies in the determination of the state of charge (SOC), the temperature and/or the current can lead to inaccuracies in the determination of the resistance. For this reason, the resistance is not determined in critical operating states, for example at temperatures below 0° C., for states of charge between 0% to 10% and 90% to 100% and in the case of low current flows, for example current flows of less than 10 A.
DE 10 2011 017 113 A1 discloses determining the aging state of a rechargeable battery during operation, e.g. of a vehicle driven by an electric machine. A Kalman filter is used herein, too, namely a triply extended Kalman filter. In the method, a first Kalman filter is used to calculate the state of charge and the fast and slow overvoltages generated by the current, and a second Kalman filter is used to calculate the internal resistance, and a third Kalman filter is used to calculate the cell capacity.
DE 10 2012 010 487 A1 discloses a method for assessing an aging state of a battery using a family of characteristic curves.
EP 1 380 849 B1 discloses a method for ascertaining the quantity of charge in a storage battery that can be drawn in the fully charged state relative to the new state by measurement of battery currents and/or battery voltages at at least two points in time before or during a rise phase and during a decay phase of a charging or discharging process by the determination of a characteristic variable for the quantity of charge that can be drawn from the relationship of at least one battery voltage value from the rise phase to at least one battery voltage value from the decay phase given an approximately identical battery current value or from the relationship of at least one battery current value from the rise phase to at least one battery current value from the decay phase given an approximately identical battery voltage value. This involves evaluating the behavior of the hysteresis obtained by plotting the battery current values and battery voltage values for preferably momentary pulsed loading in the charging or discharging direction in order to deduce from this the quantity of charge that can be drawn from the storage battery.
DE 10 2010 043 870 A1 discloses a converter control unit which responds to a command from a start determination unit to the effect of controlling a converter in such a way that a ripple current is generated at a rechargeable battery. A storage unit stores a family of characteristic curves defining a correlative relationship between the temperature and the current of the rechargeable battery and the internal resistance. An estimation unit estimates a value of an internal resistance of the rechargeable battery on the basis of each detection value of the temperature and current and the family of characteristic curves stored in the storage unit.
DE 10 2012 022 458 A1 discloses a method for monitoring an energy store, wherein operating parameters and/or operating states of the energy store are detected by sensor means and a present relative capacity state of the energy store is ascertained by means of the operating parameters and/or operating states from a first family of characteristic curves assigned to the energy store or by a first calculation function assigned to the energy store. Furthermore, a present relative energy state of the energy store is ascertained by means of the capacity state from a second family of characteristic curves assigned to the energy store or by a second calculation function assigned to the energy store.